1. Field of the Invention
The present invention relates to a coating type magnetic recording medium having a multilayer structure.
2. Description of the Prior Art
In a coating-type magnetic recording medium composed of a magnetic layer formed on a substrate by coating the substrate with a coating film of magnetic particles dispersed in a binder resin, it is desirable for the magnetic layer to have a smaller thickness to obtain low-noise, high-output characteristics. To accomplish this, coating type magnetic recording media having a multilayer structure have been proposed comprising forming between the magnetic layer and the substrate a non-magnetic coating layer obtained by dispersing non-magnetic particles in a binder resin. In this specification, the non-magnetic layer is also referred to as "lower layer" and the magnetic layer as "upper layer."
As the non-magnetic particles used to form the lower layer, there are mainly used spherical particles of titanium oxide or acicular particles of iron oxide. This type of multilayer magnetic recording medium is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 (1988), Japanese Patent Application Laid-Open (KOKAI) No. 4-167225 (1992) Japanese Patent Application Laid-Open (KOKAI) No. 6-60362 (1994) and Japanese Patent Application Laid-Open (KOKAI) No. 6-131653 (1994). It is also proposed in Japanese Patent Application Laid-Open (KOKAI) No. 4-167225 (1992), Japanese Patent Application Laid-Open (KOKAI) No. 6-139553 (1994), Japanese Patent Application Laid-Open (KOKAI) No. 6-215360 (1994), Japanese Patent Application Laid-Open (KOKAI) No. 7-78331 (1995), Japanese Patent Application Laid-Open (KOKAI) No. 7-105530 (1995), Japanese Patent Application Laid-Open (KOKAI) No. 7-182649 (1995), Japanese Patent Application Laid-Open (KOKAI) No. 7-282443 (1995), Japanese Patent Application Laid-Open (KOKAI) No. 7-326037 (1995) and Japanese Patent Application Laid-Open (KOKAI) No. 7-334835 (1995), among others. These describe characteristic values obtained when acicular hematite and the like are employed as the non-magnetic particles of the lower layer. That iron oxide hydrate and the like can be used for the lower layer is also stated, but characteristic values thereof are not included.
With respect to a magnetic recording medium having a multilayer structure, there is no actual record of FeOOH particles being used for the lower layer, neither do the above publications describe any specific examples or properties relating to when FeOOH particles are used for the lower layer. As such, there are many unknowns concerning which iron oxide hydrate will exhibit the intended functions when used as the particles of the lower layer of a magnetic recording medium. FeOOH is usually produced by oxidizing a suspension of Fe(OH).sub.2, but it is well known than even slight variation in oxidizing conditions can result in different phases, and differences in properties and morphology. Clearly, therefore, not all known iron oxide hydrates have the right properties for use as lower layer particles.
The magnetic upper layer of a magnetic recording medium having a multilayer structure is required to be as thin as possible and to have a high density. This means that the magnetic particles have to be finer and have superior magnetic properties, and have excellent dispersibility so as not to degrade the resinous binder. When suitably combined with the lower layer, it is also required for the product medium to have excellent strength, storage-stability and weather resistance, and excellent surface smoothness, while retaining the excellent magnetic qualities.
For applying FeOOH to form the particles used in the lower layer to obtain a magnetic recording medium having a multilayer structure that exhibits excellent magnetic properties, surface qualities, strength, weather resistance and so forth, in accordance with the present invention, it was necessary to clarify what effect the chemical and physical properties and shape of the particles would have on the surface smoothness, strength, magnetic properties, and weather resistance of the magnetic recording medium, and what were the most suitable magnetic particles for the upper layer that is combined with the lower layer.